The present invention relates to a stabilized flexible pipe for transporting hydrocarbons. More particularly, the invention relates to the armor wires of the flexible pipe that make it possible in particular to limit the lateral buckling of the tensile armor plies of the flexible pipe.
Flexible pipes as defined in American Petroleum Institute Recommendation API 17J generally comprise one or more polymeric layers and metal reinforcing layers such as tensile armor plies, a carcass and/or a pressure vault. The nature, number, dimensions and organization of these layers are essentially tied to the conditions of use of the flexible pipes in question and to their installation, as is defined in the API 17J. Recommendation.
When a flexible pipe, whatever its nature, is subjected to an external pressure Pe higher than the internal pressure, an axial compression, known as the inverse end effect, is produced. The inverse end effect has a tendency to produce a longitudinal compressive force in the armors and to shorten the length of the flexible pipe. In addition, the flexible pipe is also subjected to dynamic stresses, especially during installation or in service in the case of risers. All of these stresses may result in damage of one or more armor plies and thus eventually degrade the flexible pipe. An armor ply is considered to be damaged when certain wires are broken and/or have undergone substantial permanent (plastic) deformation and/or are overlapped.
A first cause of damage of the armors is an excessive stress state resulting from excessively large forces and/or deformations resulting in the rupture and/or plastic deformation of the wires. This mode of degradation may occur in the event of an inverse end effect, but also in other situations.
A second cause of damage is buckling, that is to say an instability phenomenon that may result in large displacements (and deformations) of the armors. This mode of degradation can exist only if there is a longitudinal compressive force in the armor wires, that is to say in particular in the inverse end effect situation. The instability occurs as soon as the axial compression exceeds a level called the critical load. This depends on the nature of the armor wire (modulus of the material, width, thickness) and on the state of all the armors (imposed deformation such as transverse deflections, rubbing on the other layers, resistance of the other layers, etc.). In general, when the critical load is reached, the armor will in fact be considered to be sound in view of other criteria such as, for example, stresses below the yield strength of the material.
Admittedly, buckling instability is one potential cause of damage, but in certain cases there may be buckling without damage to the armors. This occurs (even though the wire is intrinsically unstable) when the amplitude of its deformations is sufficiently limited so that a state of degradation is reached (for example by the yield strength being exceeded). Such a limitation in the deformations may be envisaged for example by bearing on the neighboring wires (limited lateral instability thanks to very small lateral clearances) or else by bearing on another layer (as in the case of radial instability). In the contrary case, since the displacements are not sufficiently limited, the lateral buckling may lead to overlapping and/or plastic deformation of the armor wires, but this is in fact only a consequence of the buckling instability.
A third cause of damage is the disorganization of the armors by overlapping of the wires. Depending on the type of flexible pipe and the dynamic stresses undergone, there are situations in which the lateral clearances between wires are taken up at least over part of the length of a laying pitch. There is therefore a threshold for the amplitude of the lateral contact forces beyond which wire overlap may occur. This threshold depends on several factors, including the geometry of the wires (thickness of the wire, concavity of the edges), the magnitude of the frictional forces, the relatively large freedom of radial movement.
To summarize, under the inverse end effect and when the pipe is subjected to dynamic stresses such as bending stresses, the armor plies of the pipe may be damaged according to the following damage modes: excessive stresses (yield strength exceeded), buckling instability and/or disorganization by overlapping. Once damage has been initiated in one of these modes for at least one wire, the equilibrium of the structure may be disturbed to the point that the other wires perish in a cascade effect, according to the same damage mode or else according to other modes.
When a flexible pipe bends, the wires of the armor plies move with respect to the subjacent core. The amplitude of these longitudinal and transverse displacements depends on many parameters, such as the bend radius, the characteristics of the armor plies (lay angle, clearance between wires, nature of the wires), the state of the external sheath (impermeable or otherwise), the applied forces (tensile and compressive forces, internal pressure, external pressure, friction forces within the structure). The displacements of these wires generally lead to clearances between wires of a given ply that have a variable amplitude along a laying pitch. In addition, the armor displacements are accompanied by wire deformations, the amplitude of which depends on the magnitude of the frictional forces.
Under certain conditions, for example with a nonimpermeable external sheath and whether the pipe is straight or bent, the armor wires may buckle in a radial mode and adopt the shape of a “bird cage”.
Another mode of damage of the tensile armors, which is due to the compressive stress that they undergo with the inverse end effect is what is called “lateral buckling”, which may occur when the flexible pipe is bent and irrespective of the state of the external sheath. The term “lateral buckling” will refer to all of the modes of lateral damage of the armor wires described above, namely excessive stresses (yield strength exceeded), lateral buckling instability and/or disorganization by overlapping.
When the external sheath is impermeable, the effects of the external pressure are liable to generate large frictional forces between the various plies of the structure. These frictional forces combined with the dynamic stresses are liable to result in migration of the wires, that is to say localized increase in the displacements, transverse deformations and lateral clearances between the wires of any one armor ply. These phenomena are thus liable to cause lateral buckling of the armor wires especially by progressive plastic deformation of their edges.
When the polymeric external sheath of the flexible pipe is nonimpermeable, whether this is intentional or else because it has been damaged for any reason, the pressure obtaining in the annulus where the tensile armors are placed is equal to the hydrostatic pressure. The migration of the armor wires under the effect of the dynamic stresses and of less friction, may result in lateral buckling or also a radial buckling phenomenon.
One of the solutions adopted, and described in patent WO 03/083343 for reducing the risks of “bird cage” radial buckling and/or lateral buckling, and also for reducing the swelling of the armors due to the inverse end effect, especially when the external sheath is nonimpermeable, is to wind tapes or reinforced layers, for example of aramid fibers, such as KEVLAR fibers, around the last armor ply and the subjacent plies. In this way, the swelling of each of the armor plies is, on the one hand, controlled while reducing, on the other hand, the risk of damage by overlapping of the armor plies. However, although this solution does solve the problems associated with radial buckling, it makes it possible only to limit the risk of lateral buckling, which persists.
In the prior art, one of the solutions used to combat lateral damage is the use of thicker armors. This solution increases the resistance of the wires to buckling instability while not penalizing them in respect of lateral damage by excessive stress due to their migration. It is also possible to reduce the risks by increasing the lay angle. Another envisaged solution consists in increasing the number of armor plies. However, all of these solutions are generally penalizing, as they are heavy, expensive and not very effective.
When an armor wire moves laterally as a result of lateral buckling, it may under certain conditions carry with it the other armor wires of the ply. The result is that the flexible pipe is if not destroyed at least rendered unusable and it has to be changed since repair of the flexible pipe is not conceivable economically speaking.